The present invention relates to devices for hydroforming metal elements and regards in particular a device of the type comprising:                a hydroforming cell, having a fixed rigid structure including:                    a base;            a top cross member surmounting the base and set at a distance therefrom; and            two shoulders, which connect the top cross member rigidly to the base; and                        a hydroforming module set within the cell between said base and said top cross member so that the vertical forces that are generated during the hydroforming process are discharged on said shoulders, which are subject to tensile stress,        
said hydroforming module comprising:                a top die and a bottom die, which can be displaced vertically with respect to one another between an open condition and a closed condition, in which they define between them a forming cavity for a metal element set between them;        means for supplying a liquid under pressure within said forming chamber; and        means for controlling the relative vertical movement of the two dies, said means comprising:                    at least one vertical hydraulic cylinder set between one of said dies and the fixed structure of the cell; and            a pair of wedge-shaped members set opposite to one another, which can be displaced by means of activator means synchronously with respect to one another on a horizontal supporting surface of the fixed structure of the cell, for causing the vertical movement of a controlled member, which is operatively connected to one of said dies.                        
A hydroforming device having all the characteristics referred to above is described and illustrated in the document No. JP-A-2004276077.
In traditional hydroforming devices, the top die and the bottom die are pressed against one another by means of a press. Obviously, since the pressure of the hydroforming liquid, during the hydroforming process is relatively high, the force that must be applied by the press becomes very high in the case of forming of large panels of sheet metal, such as for example panels of sheet metal for the bodies of motor vehicles. Consequently, said conventional devices entail the use of very powerful presses of very large dimensions. Said drawback exists also in the case of the device illustrated in the document No. JP-A-2004276077 in so far as, in said device, the aforesaid wedge-shaped members are used for setting the top die and the bottom die up against one another, whilst two vertical hydraulic cylinders are provided that are kept active to resist the high vertical force that is generated during the operation of hydroforming as a result of the pressure of the hydroforming liquid between the two dies.